Wood drying method



Jan. 1, 1963 Filed Sept. 24, 1958 LL & &

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INVENTOR Melvin R. Knudson y Delbert 6. Bowen Arforneys Jan. 1, 1963 M. R. KNUDSON ETAL 3,070,896

woob DRYING METHOD 4 Sheets-Sheet 2 Filed Sept. 24, 1958 JmK m .a f m @m A e Mum Jan. 1, 1963 M. R. KNUDSON ETAL WOOD DRYING METHOD Filed Sept. 24, 1958 4 SheetsSheet 5 mnm R M 5 w VKfl n nr Mm m d MOMv Nn W450 Oh M. R. KNUDSON ETAL 3,070,896

Jan. 1, 1963 WOOD DRYING METHOD 4 Sheets-Sheet 4 Filed Sept. 24, 1958 DRYING TIME HOURS T mm m mm m M0 UP. TE 56 m ML |.l| v m m D N H K I 0 0 0 0 0 5 4 3 2 Fig/O RADIAL DISTANCE FROM SURFACE, INCHES INVENTOR Melvin R Knudson Delbert 6. Bowen 35%,, /imw Aflorneys Patented Jan. 1, 1963 3,070,896 WOOD DRYING METHOD Melvin R. Knudsen and Delbert G. Bowen, Libby, Mont., assignors to St. Regis Paper Company, New York, N.Y., a corporation of New York Filed Sept. 24,1958, Ser. No. 763,062 7 Claims. (Cl. 34-26) This invention relates to the drying and seasoning of wood, and more particularly to the procedures and means for rapidly drying wood to achieve improved characteristics, some of which are normally obtained only by very prolonged air seasoning.

Drying or seasoning of wood is a very important step in its preparation for re-manufacture and may be done either before or after a log is cut into boards. Much water is removed during the process, both in the lumina of the cells and in the cell walls themselves, causing a considerable amount of shrinkage of the wood. This shrinkage is generally greater tangentially than in other dimensions so that the wood on drying tends to check or crack longitudinally, especially in large round pieces of wood. The rate of drying is quite critical since an improper rate may cause the wood to crack or rupture check so severely that very wide crack may result, extending frequently into the pith of the wood. On the other hand, undried or green wood is entirely unsatisfactory for most purposes, it will twist, warp and crack as it dries under uncontrolled conditions in service, cannot be easily finished or glued, and is more subject to attacks of fungi and boring insects.

There are two general methods of seasoning round, or sawn wood; air drying and kiln drying. The most satisfactory results, on round material, are obtained by air drying, or so-called air seasoning of the wood, in which the Wood is arranged in piles for maximum exposure to the air. The drying of wood in this way may require a month or more for sawn boards an inch thick and sometimes as long as two years for large poles. Naturally the climate greatly affects the time required and uniform drying conditions are difiicult to establish, particularly in the northern latitudes where seasonal variations are extreme. The material being seasoned may be periodically covered by ice and snow or drenched by heavy rains and thereby subject to fluctuating moisture contents, and also subject to fluctuating wood temperatures. Variable weather conditions therefore tend to olfset the benefits of air seasoning. Despite these drawbacks, properly air seasoned wood has many properties which give it special value, and is characterized by a very even moisture gradient extending into the pith or core of the wood. It retains its inherent properties of great strength and incompressibility, remains flexible and elastic, with the ability to recover after bending.

Because of the excessive time required for air seasoning, various procedures have been developed to make it possible to dry wood in a few days instead of months or years. The most common method is known as kilndrying or kiln-seasoning. However, kiln-drying and similar procedures have never successfully reproduced the characteristics of air seasoning. The drying frequently produces rupture checks or cracks, particularly in round material. Abnormal moisture gradients are also generally produced, and tend to materially reduce the desired properties of the wood. Such gradients are caused by a retarded drying at the surface with little drying of the interior of the wood, as compared to air seasoning, and frequently are evidenced by hard crusts or shells near the surface of the Wood.

In general, it is an object of the present invention to provide a new and economical method of drying and seasoning wood by which all of the advantages normally attributed to air seasoning can be obtained in a fraction of the time, and without the many disadvantages inherent in such processing.

It is another object of the invention to provide a method by which a uniform moisture gradient, from core to exterior, can be consistently obtained in artificially dried wood.

Another object is to effectively season wood, particularly round material, in a period of about 50 hours or less, and without discoloration of the wood.

Another object of the invention is to provide an improved apparatus for carrying out such method.

A further object of the invention is to provide a procedure and apparatus for drying wood that is readily adapted to existing manufacturing facilities, is simple to employ, and is relatively inexpensive of operation.

Other objects and advantages of the invention will beapparent from the following description and from the drawings in which FIGURE 1 is a view in longitudinal section of an improved timber drying and seasoning system embodying my invention;

FIGURE 2 is a view in horizontal section of the system of FIGURE 1;

FIGURE 3 is an enlarged detail view along the line 3--3 of FIGURE 2;

FIGURE 4 is a view in vertical section along the line 4-4 of FIGURE 3;

FIGURE 5 is a like view along the line 55 of FIGURE 3;

FIGURE 6 is a detail view in side elevation of a tram car, useful in carrying out the invention;

FIGURE 7 is a like view in front elevation of the tram car, indicating the manner in which timber is loaded on the cars;

FIGURE 8 is a schematic representation of a control apparatus suitable for carrying out the invention; and

FIGURES 9 and 10 are graphic illustrations of drying results made possible by the improved procedures of the invention.

Generally stated, the present invention is based on the discovery that if timber, such as poles, cross arms, piling, cross ties and the like, is subjected to an atmosphere having a controlled relatively high temperature and low humidity, a fine, hair-checking pattern will develop over its entire surface. This pattern has the advantage of preventing large cracks or rupture checks from forming on subsequent rapid drying of the timber to a desired low moisture content. I have found that best results are obtained with a circulation of air maintained at a temperature between about 200 F. and 240 F. and a relative humidity below about 30%. These conditions of atmosphere permit the creation of the hair-checking pattern in a very short period of time, definitely within the first four hours of drying, and apparently within the first 30 to 45 minutes of the drying process.

The indicated conditions of drying and the establishment of the checking pattern are quite critical to the process, as apparently the myriad of tiny cracks tend to absorb all shrinking of the material as it dries without localization of the shrinkage in a few large deterimental checks or cracks. The drying also establishes a very even, moisture gradient from the exterior of the timber to the core, and permits a maximum content of about 25% (dry weight basis) to be achieved at a depth of about 1% inches within a period of less than about 50 hours. This moisture gradient pattern is quite similar to that obtained with air seasoning, yet no graying or discoloration is produced. The treated timber also has advantageous characteristics of uniformity and stability of moisture content incapable of being obtained by normal air seasoning processes.

In carrying out the invention, I prefer to employ a relatively elongated tunnel-like building 6, having vertical side walls 8, a floor and a roof 12. The building can be separated into upper and lower passages 14 and 16 by means of an elongated horizontal baffle 18 which preferably is of less length than the building so as to provide air circulation passages about its ends. Tracks 20 can be provided upon the fioor to permit stacks or packages of Wood or timber as indicated at 22, to be moved into position in the lower drying passages 16, for example on tram cars 24. Doors 26 are provided in at least one end of the tunnel to permit the tunnel to; be loaded and unloaded. In general, a plurality of spaced stacks of timber are positioned in the lower passage 16, and the doors closed to make the lower passage part of a normally closed circulating system including both the passages 14, 16 and the passages about the ends of baffle 18.

The amount of timber treated in the drying chamber 6 will depend partly on its length and partly on the disposition of the timber following treatment. For example, if the material is to be subsequently preservatively treated, the drying chamber 6 can be built to accommodate a volume of material equal to the volume of the preservative treating retorts. Such an arrangement makes possible an immediate preservative treatment of uniformly warm, dry, timber, and permits many advantages both in economics of operation and success of treatment as will appear.

In accordance with the invention, air of controlled temperature and humidity is caused to circulate through the timber contained in the lower passage 16 throughout the period of a drying cycle. Means for accomplishing this controlled circulation include a fan or other suitable means 28 for moving air through the passage 16 and back through the return passage 14, heating elements 32 for heating the air, and inlet ducts 34 and vents 36 for regulating the humidity of the air. This circulation indicated in the drawings by the arrows 30, can be in either direction and preferably is periodically reversed under the control of automatic timing devices.

In general, humidity is controlled by removing warm moist portions of the circulating air and replacing such air with fresh dry air, as necessary, and as determined by a suitable temperature and humidity controller 38. As will appear, the controller 38 can be of a recording type responsive to relative humidity as determined by wet and dry bulb measuring devices at '40 and 42. The temperature of the circulating air can be controlled bya thermostatic valve regulating the flow of heating fluid to the heating elements 30, and in response to dry bulb temperatures as determined by the controller 38. Any suitable means can be employed for heating the circulating air. By way of illustration, heating may be accomplished by a plurality of fin-tube heating elements 46 supplied with steam preferably under pressure, through the valves 44.

Although control over the temperature and humidity of the circulating air is critical to the processing in accordance with the invention, the means for carrying out such control may be any of several well'known standard types. As previously indicated, the controller device 38 is preferably of the temperature and humidity recording type. In general, the function of the control apparatus is to maintain the temperature and relative humidity of the air circulating through the drying passages 16 within the critical operating ranges defined above. More specifically, the valves 44 are directly responsive to the dry bulb temperatures sampled by the controller to admit steam to the heating elements, should the temperature of the circulating air fall below a predetermined value, say 215 F. They can also be responsive to an upper limit, say 235 F., the effect of the controller being to maintain the temperature of the circulating air within the desired range of from 210 to A; 240 F. The controller further operates to regulate relative humidity by opening the inlet ducts 34 and the vent outlets 36 in response to an upper limit of relative humidity existing within the drying chamber, say 25%, as recorded by the wet and dry bulb recorders. It is contemplated that operation of the control openings 34 and 36 will be periodic and that they may operate simultaneously or alternately, as may be desirable in a particular circumstance. It will be understood that particular temperature and humidity controllers of the type referred to are well known in the art, and consequently no detailed description is necessary.

FIGURE 8 shows schematically one suitable control system useful in carrying out the invention. As illustrated, the controller 38 has a dry bulb recorder 39 responsive to the dry bulbs at 40 and 42, and adapted to actuate the valves 44 supplying steam to the heating elements through the valve 41. The controller also has a wet and dry bulb recorder 43 responsive to the wet bulb 40a and dry bulbs 40 and 42, and adapted to actuate the ventilation system or ducts 34, 36 through the valve 45.

To explain the operation of the illustrated control system, it may be assumed that the dry bulb recorder 39 is set to operate Within a specific temperature range determined by the limit mechanisms 47, say 215 to 235 F. Thus if the recorder 39 moves above 235 F., the recorder acts to close valve 41 causing the steam valves 44 to close. Upon a subsequent decline in the dry bulb temperature of the circulating air, causing the recorder 39 to fall below 215 F., valve 41 functions to open valves 44 to again admit steam to the coils 32. In similar fashion it may be assumed that the wet bulb recorder 43 is set to operate at a depression reading between the wet bulb at 46a and the dry bulb at 40 or 42 (whichever is higher in temperature) corresponding to a relative humidity higher than that for which the controlling instrument is set, say 25%. Thus so long as the wet and dry bulb recorders indicate a relative humidity below 25%, the valve 45 will remain closed, and the ducts 34 and 36 will also remain closed. When the depression reading between the recorders 39 and 43 is such that a relative humidity higher than 25% is indicated, the recorder 38 acts to open the valve '45 to admit compressed air to the air operated diaphragm motors 51. These air motors in turn actuate the lever attachments 53 and pull rods 55 for the automatic operation of the inlet ducts 34 to admit fresh low humidity air, and for the roof vents 36 to discharge chamber high humidity air to the atmosphere. The effect of admitting fresh air and exhausting high humidity air is to lower the humidity in the chamber until the depression reading between the recording instruments 39 and 43 is such as to register a relative humidity lower than 25 At this point the recorder again closes the valve 45 to close the ventilating system of ducts 34, 36. It will be understood that the wet bulb temperature of the air is directly proportional to the total heat content (enthalpy) of the circulating air. In other words, when the air loses heat, the wet bulb temperature goes down. Conversely when the air gains heat, the wet bulb temperature goes up. Therefore, venting of the inside air not only reduces humidity but also reduces the heat content (enthalpy) of the inside air. This heat content is restored by the steam coils. Accordingly the control system of the present invention permits simultaneous control both of the humidity and of the heat content of the circulating drying air.

In the drawings air circulation is indicated by the arrows 30 as being in a generally clockwise direction. It will be apparent however that as the air moves through the drying passage 16, evaporation of moisture from the timber will cause it to have a somewhat lower temperature and corresponding higher humidity as it approoaches the left hand end of the tunnel. To remedy the nonuniform drying conditions thus produced, it is desirable that the circulation of air through the tunnel be periodically reversed. Such reversal can be easily accomplished by use of reversible motors 48 as the power source for the fans 28. As previously indicated this reversal of air circulation can be automatically controlled by a timer mechanism, which may initiate the reversal at any desired interval, say every half hour.

In the drying of the lumber or timber it is desirable that a maximum contact of a load with the circulating gases be obtained. Such contact is facilitated in the apparatus illustrated by the provision of adjustable air restrictors 50 positioned in the path of air flow at spaced points Within the passage 16. These restrictors can conveniently be hollow wooden frames constructed to provide generally triangular baffle sections 52 at corner positions within the passage 16, as indicated in FIGURE 4, and preferably are mounted for sliding movements longitudinally of the tunnel. The function of the baffies 52 is to create zones of turbulence or high pressure adjacent the outer walls of the chamber while simultaneously estab lishing zones of relatively lower pressure within the material. The effect is to draw the drying air to the zone of low pressure within the interior of the load being dried. The precise positioning of the bafiies is to some extent an engineering choice depending upon the length and relative positioning of the timber stacks 22 within the passage 16. However, it is of advantage if a restrictor or series of restrictors 50 is so positioned with respect to the load, or a particular package or stack 22, that somewhat more than half the length of the load or stack is confined within the zone or zones encompassed by the restrictors.

The operation of the drying chamber 6 can be as follows: The doors 26 are opened and packages 22 of timber are moved into position within the drying passage 16, as illustrated in FIGURE 1. The doors are closed to form a closed circulating system within the chamber 6, and the fans 28 started to initially circulate air in a clockwise direction through the timber as indicated by the arrows 30. Preferably the air is circulated to provide an air velocity through the passage 16 of between about 1800 to 2000 feet per minute. At suitable intervals during the drying cycle, say every half hour, the direction of air flow can be reversed so drying conditions within the tunnel can be maintained substantially uniform. The temperature of the circulating air is maintained by the controller 38 and steam valve 44 at a temperature between about 210 to 240 F. (optimum 230). The relative humidity of the circulating air is maintained within the range of 15% to 30% by means of the controller and the vent openings 34 and 36. Drying is continued in accordance with the program settled on for the particular species, and/or form of timber undergoing drying until a predetermined condition of dryness has been attained. For example, 12 inch diameter poles of one of the longest seasoning woods, lodgepole pine, can be dried in a period of about 36 hours to a maximum moisture content of approximately 25% at a depth of 1 and inches, measured radially into the poles.

The advantages of the above processing can be summarized as follows: Large quantities of timber ranging up to a charge of several thousand cubic feet can be rapidly dried in a period of hours rather than in a period of months or years. The dried timber has a very even moisture gradient from surface to pith quite similar to that obtained by air seasoning, and possesses excellent strength and incompressibility while retaining a high degree of flexibility. The drying establishes a fine hairehecking pattern uniformly distributed over the entire surface area of the material within at least the first four hours of drying time, and thereby prevents the formation of large cracks or rupture checks so characteristic of kilndried and much of air seasoned timber. The wood retains substantially its original light color without developing the characteristic grey color normally resulting from air seasoning, and therefore makes possible the production of a unique light colored preservatively treated material. A very high utilization of standing timber is made possible since the need for large inventories of pole material in an air seasoning yard is eliminated. Of particular importance is the fact that timber having uniform quality, regardless of seasoning can be obtained, providing a heretofore unequalled versatility in filling timber orders.

One further advantage is that subsequent preservation treatment of timber is greatly facilitated and simplified. for example, as in the treatment of poles, piling, posts, railroad ties, and construction timber and lumber, with creosote, penta-chlorophenol, petroleum oil, etc. Such treatment heretofore has been subject to a number of difficulties traceable, in general, to a nonuniformity of the material being treated. These difiiculties usually resolve themselves into either improper or insufficient penetration or retention of preservatives in the wood, or into an excessive loss or dilution of expensive preservative fluid. With the processing in accordance with the invention, the uniform characteristics of the dried wood permits a more effective uniform penetration and retention of the preservative in the timber, and thus a more efficient utilization of the preservative fluid than has been possible with prior processes. Much shorter retort times are required and the preserved timber is of a lighter color, making it more commercially acceptable. Moreover, the processed timber is adapted to be inserted warm directly into the preservative treating chamber without rehandling of the material and with consequent saving in both labor and conditioning costs.

The following examples are illustrative of the practice of the invention.

Example 1 Employing apparatus of the type illustrated in the drawing, charges of small barn-pole size lodgepole pine poles, averaging two thousand cubic feet of material per charge are dried from an average moisture content of 120%, measured in the outer 1 1" annuli, down a final average moisture content in this zone of 18%. This outer annuli encompasses approximately 70% of the total volume of the material being dried. The average weight of the material in the charge at 120% moisture content is about 50 pounds per cubic foot. The average weight of the same material at a moisture content of 18% is about 30 pounds per cubic foot. A weight reduction of 20 pounds per cubic foot resulting from drying therefore amounts to a removal of a total of 28,000 pounds of water. It should be noted that a weight reduction of this nature is seldom achieved through employment of facilities and equipment available at the present time in the wood preserving industry. This weight reduction is only comparable to results secured from long periods of air seasoning under more or less ideal weather conditions.

During the drying the temperature of the circulating air is maintained at about 230 F. and the relative humidity at about 20%. Approximately 43,000 cubic feet per minute of fresh air is needed in the drying, with air velocities averaging about 1900 feet per minute. Drying is accomplished to the desired moisture content in an average period of about 36 hours as illustrated in the representative drying curve of FIGURE 6. This curve indicates a relatively uniform drying rate varying from about 725 to 900 pounds of water removed per hour.

The dried timber is observed to have over its entire surface a fine hair-checking pattern of uniform distribution. The dry, seasoned timber also has a light natural color, characteristic of freshly cut, unseasoned material.

Similar results have also been obtained in the drying of western larch, Douglas fir, white fir and hemlock. The range of material sizes varies from barn pole sizes up to poles feet or more in length.

Example 2 Employing timber processed as in Example 1, random samples are tested to determine moisture content and a gradient from the exterior to the core of poles averaging in diameter from to 12". Results of the test on lodgepole'pine poles are indicated graphically in FIGURE 10. These results indicate a uniform moisture gradient characteristic of air seasoned timber, obtained however withina drying period of about 36 hours or less.

' Example 3 Timber dried as in Example 1 is introduced while still warm (approximate temperature 150 F.) into a retort forpreservatively treating the timber. Preservative liquid comprising coal tar creosote, or pentachlorophenol solution, at a temperature of about 195 F., is then caused to completely surround the timber in theretort, with contact and pressure of about 100 p.s.i. being maintained for a period of from 1 to 3 hours. The retort is then drained of preservative, and the remaining steps of a normal treating cycle completed. Upon removal from the treating retort, the material is found to have a uniform retention, and a uniform penetration, of the preservative to a depth of about 1%". The treated timber is also observed to have a light color.

To those skilled in the art to which this invention relates, many variations and widely difiering applications and embodiments of the invention will suggest themselves without departing from the spirit and scope of the invention. For example, it is contemplated that preservatively treated timber may again be dried in the drying chamber, under like conditions, to impart enhanced resistance to bleeding and other desirable characteristics to the timber, or a series of treatments in the drying chamber may be employed with treated or untreated material. Accordingly it will be understood that the disclosures herein are purely illustrative and not intended to be in any sense limiting.

We claim:

1. In a process of drying Wood, the steps of enclosing the wood in a chamber forming part of a normally closed circulating system, admitting air of low relative humidity to the chamber, causing such air to circulate through the zone of enclosure of said wood, heating the air to maintain a circulating air temperature within the range from about 210 F. to 240 F., such heating further lowering the relative humidity of the circulating air to thereby induce evaporation of moisture from said wood, continuously sampling the relative humidity of the circulating air, withdrawing moisture laden air from the chamber when its relative humidity exceeds a value within the range of about 15% to simultaneously replacing the withdrawn air with additional air of low humidity, and continuing the drying process for a period of less than about 50 hours to achieve a substantially uniform moisture gradient in the wood from surface to pith.

2. The method of claim 1 wherein the direction of air circulation is caused to reverse periodically;

3. The method of claim 2 wherein said reversal of direction occurs about twice each hour.

4. The method of claim 1 wherein the drying of said wood is allowed to proceed until a moisture content of about 25%, on 'a dry weight basis, is achieved at a distance of about 1 and inches below the surface of the wood.

5. The method of claim 1 wherein the wood being dried is in the form of cylindrical poles.

6. The method of claim 1 wherein the wood being driedis in the form of 'sawn material.

7. In a process of drying wood, the steps of enclosing the wood in a chamber forming part of a normally closed circulating system, admitting air of low relative humidity to the chamber, causing such air to circulate through the zone of enclosure of said wood,heating the air to maintain a circulating air temperature within the range of from about 210 F. to 240 F.,- such heating further lowering the relative humidity of the circulating air to thereby induce evaporation of moisture from said wood, continuously sampling the relative humidity of the circulating air, withdrawing moisture laden air from the chamber when its relative humidity exceeds a value within the range of about 15% to 30%, simultaneously introducing additional air of low relative humidity to the chamber, continuing the drying process for a period of less than about 50 hours-to achieve a substantially uniform n oisture gradientin the wood from surface to pith, said moisture gradient 'being similar to that obtained by prolonged air seasoning, removing the dried wood from said chamber and treating the same while still warm with a preservative solution, thereafter reintroducing the dried treated wood to said drying chamber and repeating the drying process under like conditions to impart enhanced resistance to bleeding of the dried treated wood.

References Cited in the file of this patent UNITED STATES PATENTS 1,677,963 Ford July 24, 1928 1,863,943 'Rubin June 21, 1932 2,064,965 Will Dec. 22, 1936 2,184,473 Scanlan Dec. 26-, 1939 2,270,815 Vaughan Jan. 20, 1942 2,273,039 Hudson Feb. 17, 1942 2,288,154 Cobb June 30, 1942 2,296,546 Toney Sept. 22, 1942 2,341,653 Ricchiardi Feb. 15, 1944 2,403,154- Simmons July 2, 1946 2,763,069 Vaughan Sept. 18, 1956 OTHER REFERENCES Kiln Drying of Lumber, A. Koehler and R. Thelen, d McGraw Hill (1926), New York.

Ape. 

1. IN A PROCESS OF DRYING WOOD, THE STEPS OF ENCLOSING THE WOOD IN A CHAMBER FORMING PART OF A NORMALLY CLOSED CIRCULATING SYSTEM, ADMITTING AIR OF LOW RELATIVE HUMIDITY TO THE CHAMBER, CAUSING SUCH AIR TO CIRCULATE THROUGH THE ZONE OF ENCLOSURE OF SAID WOOD, HEATING THE AIR TO MAIN- 